<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>

    <style>

        *{
            margin: 0;
            padding: 0;
        }

        .box{
            width: 100%;
            height: 100%;
            background-color: aqua;
        }
        html,body{
            height: 100%;
        }

        .child1{
             background: red;
             height: 50%;
        }
        .child2{
             background: blue;
             height: 50%;
        }
    </style>
</head>
<body>
    <div class="box">
        <div class="child1"></div>
        <div class="child2"></div>

    </div>

</body>
</html>